It's great that you have the opportunity to acquire a Neve audience reaction mixer for a good deal and plan on modding it to suit your needs.
Regarding your question about using it as a preamp, it's important to note that the Neve audience reaction mixer was not originally designed as a standalone preamp, so it may not necessarily function optimally in that capacity. However, the T1452 followed by a B338 opamp and VTB 1148 output transformer is indeed a complete Neve preamp circuit, and it's possible that it could sound good at line level as well.